Geologists build model for next big Bay Area quake

A new model re-creating a devastating San Francisco earthquake gives scientists clues into how the earth may behave in the next big Bay Area shake-up.

A few weeks ahead of a conference commemorating the 1906 San Francisco earthquake, scientists from the United States Geological Survey and Stanford University released data from a 3D simulation of the quake that rocked the region 100 years ago. The data and computer simulations, in turn, will help geologists, civil engineers and others to develop strategies for the next one.

The 1906 quake probably had a magnitude of 7.8 to 7.9 and rumbled 300 miles along the San Andreas fault, all the way up to Cape Mendocino, said Greg Beroza, a professor of geophysics at Stanford. Scientists don't have an easy way to directly measure the magnitude of that quake because the Richter magnitude scale used today was not developed until 1935.

The earth moved about two feet in a horizontal direction. Although the quake occurred roughly 10 miles below the surface of the earth, it took only about three seconds for the ground to begin shaking. The leading edge of the quake--the first waves emanating from it--traveled at 14,000 miles per hour, while the shaking behind it traveled at 8,000 miles per hour.

"It was quite a big quake," Beroza said. "We expect many of the areas hit hard along the San Andreas fault in 1906 to be hit hard again" in the next major quake.

The 3D model like the one used in the study are helping geologists better map the seismic terrain of a region. "We can predict where those hot spots are going to be and concentrate retrofitting activity on them," said Mary Lou Zoback, a senior research scientist with the USGS. The Santa Cruz, Calif., area, which juts up against a mountain range, and the neighborhoods built on landfill in San Francisco typically suffer significant damage in a San Andreas quake.

The models, she added, also show that "you can't get in your car and outrun one of these things."

The researchers will next likely simulate damage from quakes along the Hayward fault, a major branch of the San Andreas fault in central California.

The conference starts in San Francisco on April 18, the 100th anniversary of the 1906 quake.